If it had the integrity to follow through the logic of its position, the government would make membership of the British National party a criminal offence. Ministers would be behaving illiberally - dangerously so, for reasons I will get to - but at least they would be demonstrating a consistency in their dealings with fascistic forces that has so far evaded them.

Instead of being honest, the political left is slowly turning Britain into a country where the state can blacklist members of a foul but legal political party. To date, it has only given itself the power to sack police and prison officers for membership of the BNP. Elsewhere, soldiers cannot take part in political activity and doctors, nurses, civil servants and teachers must not allow their beliefs to compromise their professional conduct, but they can keep their allegiances to themselves.

Unfortunately, the sensible compromise that extremists must give proof positive that they are not fit to carry out their duties before the authorities deprive them of their livelihoods seems too sensible to last.

The teachers' unions want BNP members out of the schools. Chris Keates of the NAS/UWT told me that heads should not have to wait for complaints from parents or colleagues. Teachers have subtle powers over pupils that outsiders can miss, she explained. They can promote favourites, ignore bullying and undermine the confidence of their targets. Waiting for normal disciplinary procedures was too great a risk. The sympathetic education secretary Ed Balls is listening to her and considering removing BNP members from classrooms and school governing bodies.

Instinctively, I want to urge him on. To my mind, any parent would be justified in taking their children out of a school that employed a BNP teacher. In any case, how many teachers are neo-fascists (no sniggering at the back please)? BNP membership records leaked last year revealed just 13 of its Aryan warriors were working in schools. Given the small numbers involved and the possibilities for undetected harm against children, my gut feeling is that we should not be too squeamish.

But the gut is not a thinking organ. If you fire police officers and teachers, then there is no reason not follow the advice of the PCS civil service union and drive BNP members out of the whole public sector and- why not? - the private sector too. If it is wrong per se to allow BNP members to arrest suspects, teach children and deal with benefit applications from the ethnic minorities, then it must be equally wrong for them to assess insurance claims or conduct job interviews.

As statements of basic principle never win you friends in England, I will state the theoretical objection that it is unjust to penalise men and women for their political views without further evidence of wrongdoing only briefly and move on to the practical difficulties.

According to its membership records, there are about 12,000 BNP members. Finding and firing them would be a task the like of which Britain has never undertaken before. As Stalin's armies imposed dictatorships across Europe, George Orwell warned the 1945 Labour government about the dangers of employing real and potential Soviet agents in the Foreign Office. It followed his advice, but outside the diplomatic corps and security services, British McCarthyism was a puny phenomenon.

Indeed, Britain in the 1950s rather proudly provided a sanctuary for Hollywood writers and directors fleeing the anti-communist campaigns of cold war America. It still allowed communists to teach students. The worst that Eric Hobsbawm could say about the state vetting he encountered was that his support for Stalin meant that he wasn't promoted up the academic ladder as quickly as he might have been. Be that as it may, he wasn't sacked or sent to prison and ended up receiving the Order of Merit from that notable socialist, Her Majesty, the Queen.

Last week, Professor Peter Hennessy confirmed the impression that the old elite was wary of purges when he released the findings of his investigations in the cold war archives. Even in the event of imminent war with Soviet Union, the police planned to detain only a few dozen enemy aliens and communist sympathisers. Half a century on, the state seems ready to move from dealing with dozens to thousands.

Assuming it can unmask them, that is. For finding out who is a BNP member is nowhere near as easy as it sounds. When the list of members appeared on the net last year, many on it complained that they had nothing to do with neo-fascism. If Labour instigates a purge of the public sector, it will need tribunals to ask the victims of dismissal: "Are you now or have you ever been a member of the British National party" and weigh the veracity of their denials.

Instead of adopting the methods of the witch-finder, ministers could try behaving like politicians. They could abandon selective anti-fascism and notice that many of the supposedly left-wing thinkers and trade union leaders who urge them to sack BNP members have been happy to share platforms with the reactionary ultras of Jamaat-i-Islami and the Muslim Brotherhood, as indeed have Jack Straw and many another Labour grandee.

Opposing sectarianism equally without regard to colour and creed would not only be principled, but would have the additional advantage of reducing racism in the white working class.

The current double standard is the result of a version of multiculturalism, which has placed a sinister and ignorant emphasis on race and religion. Immigrants, and particularly their children, have not been acknowledged as full British citizens, but stuffed into boxes labelled "the blacks", "the Muslims", "the Hindus" and seen everyone from the local council to the BBC treat unelected and sectarian "community leaders" as their authentic representatives. Idiotically, the proponents of multiculturalism forget that the working class could play the same game, label itself as "the whites" and insist that society must uncritically "celebrate its diversity" as well. Given the scale of the folly, we should be grateful that the BNP vote remains so small.

The chances of ministers correcting past errors are long. But I live in the hope that in its dying days, Labour will grasp that you don't defeat opponents by briefing lawyers and quangocrats, but by fighting the battle of ideas as if you meant to win it.